Site wide activity is “added” or “recorded” by performing actions within BuddyPress, or by outside plugins adding those entries into the activity table.
Depending which version of BP you’re using, check out how the function bp_activity_add(trunk)/bp_activity_record(stable) works. You would need to “import” the feed entry by entry into the activity table using one of those functions.
Rohan,
Do let us know if you succeed at this. This would be a good interim solution until someone can cook up full external blogs functionality. Thanks.